1. Field of the Invention
This invention relates to a laminated structure. More particularly, this invention relates to a laminated structure comprising a plurality of polymeric layers.
2. Prior Art
Laminated structures comprising a plurality of polymeric layers are, of course, well known in the prior art. Such structures are taught in U.S. Pat. Nos. 4,058,647; 4,198,327; 4,332,858; 4,341,837 and 4,588,648 and in Japanese Patent Application No. Sho58 [1983]-13242. In general, laminated structures comprising a plurality of polymeric layers are fabricated for the purpose of obtaining a single structure having the advantages of each of the separate layers. Before such structures will perform successfully, however, it is important that the various layers be suitably adhered one to the other so as to avoid separation during use. Heretofore, several methods have been proposed for effecting such suitable adherence of layers. For example, U.S. Pat. No. 4,058,647 teaches that certain polymeric materials, including ethylene-vinyl alcohol copolymers, may be laminated with a polymeric composition comprising both a modified and an unmodified polyolefin and a rubber component. Similarly, U.S. Pat. No. 4,198,327 teaches that various polymeric materials, including polycarbonates and ethylene-vinyl alcohol copolymers, may be satisfactorily bonded together with a composition comprising a carboxylated polyolefin and a hydrocarbon elastomer. U.S. Pat. Nos. 4,332,858 and 4,341,837 teach that various polymeric materials including olefin homopolymers, olefin copolymers such as ethylene and vinyl alcohol, polycarbonates and the like may be adhered with a modified, but unhydrogenated, block copolymer such as a maleated block copolymer of styrene and butadiene. The modified block copolymers taught by these patents, however, are subject to degradation at temperatures above about 205.degree. C. and cannot effectively be used when the preparation or subsequent treatment or use of the laminates requires temperatures above this value. As a result, the modified, but unhydrogenated, styrene-butadiene block copolymers cannot effectively be used in the preparation of laminates via coextrusion with a relatively broad range of polymeric materials since temperatures above 205.degree. C. are frequently required. Moreover, the resulting laminates cannot be finished or used at temperatures as high as those frequently desired for certain laminate applications such as thermoforming, retorting, hot filling and the like. U.S. Pat. No. 4,588,648 teaches that certain polymeric materials such as polypropylene and ethylene/vinyl alcohol copolymers may be laminated with an adhesive layer comprising a grafted copolymer of an olefin and maleic anhydride and an ungrafted polypropylene. As is well known, tie layers of this type are particularly effective in the preparation of various laminates but layers of certain such laminates are not sufficiently compatible to permit the recycle of any excess of such laminate; i.e., laminate remaining after a pattern has been cut or otherwise separated from the laminated structure. This, then, significantly restricts the range of polymeric materials that can be laminated, particularly when high costs are associated with the excess waste. Japanese Patent Application No. Sho58 [1983]-13242 teaches that various polymeric materials such as olefin homopolymers, olefin copolymers such as ethylene/vinyl alcohol, polycarbonates and the like may be tied in a laminated structure with an adhesive comprising a modified crystalline olefin-based polymer, which adhesives may also comprise an unhydrogenated block copolymer of styrene and butadiene. This particular adhesive, then, would to some extent, at least, exhibit the compatibility problems known to be associated with modified olefin polymers and also be subject to the temperature restrictions known to be associated with adhesives comprising styrene-butadiene block copolymers containing significant ethylenic unsaturation in the butadiene polymer block. In light of these deficiencies, then, the need for a laminated structure which is compatible (i.e., can be recycled) and which can be used without severe temperature limitations is believed to be readily apparent.
Adhesive and similar compositions which may, at least, comprise a hydrogenated block copolymer containing at least one monoalkenyl aromatic hydrocarbon block and at least one conjugated diolefin block, which adhesive compositions would be useful over a broader range of temperatures, are, of course, known in the prior art. Polymeric compositions which may contain a hydrogenated block copolymer are taught, for example, in U.S. Patent Nos. 3,607,977 and 3,970,711 and in European Patent Application No. 0169987. U.S. Patent No. 3,607,977 teaches compositions comprising a block copolymer containing at least two monoalkenyl aromatic hydrocarbon polymer blocks and at least one conjugated diolefin polymer block and a block copolymer comprising at least one monoalkenyl aromatic hydrocarbon polymer block and at least one conjugated diolefin block wherein at least 10% of the initial unsaturation is reacted so as to incorporate a polar group. The block copolymers may, independently, be neat or hydrogenated. The polar group may, inter alia, be an oxygen containing group such as a carboxyl group. Compositions comprising an oxygen containing group are said to be especially useful as adhesives, particularly for polar textile such as cellulose. U.S. Patent 3,970,771 teaches a primer coating composition comprising a selectively hydrogenated block copolymer having at least two monoalkenyl aromatic hydrocarbon polymer blocks and at least one hydrogenated polymer block of a conjugated diolefin, a substantial portion of a resin compatible with the monoalkenyl aromatic hydrocarbon polymer blocks and, optionally, a substantial portion of certain carboxylated resins. The primer is useful for bonding various coating materials to substrates having low energy surfaces such as substrates which are essentially hydrocarbon. European Patent Application 0169987 teaches an adhesive composition comprising a copolymer of an olefin and an ethylenically unsaturated carboxylic acid, and, optionally, an elastomer which may be a hydrogenated block copolymer of a diene and a vinyl aromatic compound. The adhesive composition may be used to bond a polymer layer such as a polyamide or polyester to a metallic screen. While certain of the adhesive compositions taught by these patents would be useful over a broader range of temperatures than those heretofore used in the preparation of polymeric laminates, particularly polymeric laminates comprising polar and non-polar polymeric layers, there is no indication that these adhesives would be useful for the preparation of such structures. The need, then, for laminated structures, particularly a laminated structure comprising a polycarbonate, which is both compatible and useful at higher temperatures continues.